1. Field of the Invention
The present invention concerns an image display control device operable to provide improved feeling contrast for input image signals, and a display device utilizing the image display control device.
2. Description of the Related Art
With the development of the information society, occasions in which users view images that are displayed on display devices are increasing. However, the display capabilities of recent display devices have not necessarily reached an adequate level, and therefore, measures are necessary for improving the apparent view-amenity better than the cases where input image signals are input as they are into a display device.
In regard to this, a reference 1 (“Gradation Conversion by a Sigmoid Function in a Digital Camera System,” Watanabe et al., Japan Hardcopy 2000 Collected Papers B-27, The Imaging Society of Japan) proposes a method of using a sigmoid function to make black (low level) blacker, make white (high level) whiter, and to expand feeling contrast to the middle level, thereby giving vividness and solidity to the image.
The sigmoid function is a nonlinear function, such as the following:y={a^(1−r)}*{x^r} (0<=x<=a)y=1−{(1−a)^(1−r)}*{(1−x)^r} (a<x<=1)
Here, the parameter “a” is the mode of the luminance histogram of input image signals and the parameter “r” is a constant.
However, the prior art described in the reference 1 has the following problems.
(Problem 1) Since the sigmoid function is nonlinear, it cannot be realized by a simple operational circuit. It is therefore not practical in realizing the function by hardware.
(Problem 2) In the reference 1, the mode of a histogram is used as a parameter of the sigmoid function. However, in a case where pictures change in a continuous manner, this parameter tends to change greatly for every frame of picture and a flicker occurs in the displayed image after conversion.
Also, in general, the Y signal of Yuv color space is used as brightness. This Y signal may thus be used in a contrast improving process.
However, the Y signal capacity of the Yuv color space is greater than the brightness component capacity of RGB color space. Therefore, when a process of increasing the Y signal is performed without special consideration and then an inverse mapping from the Yuv color space to the RGB color space is performed, a result that exceeds the brightness component capacity of the RGB color space, that is, a result that cannot be displayed, actually may be obtained. Consequently, color clipping may occur.
For example, when a signal of R=0%, G=0%, and B=80% in the RGB color space is mapped to the Yuv color space, the brightness is determined as luminance Y=24%. Further, if a process of doubling the brightness (which is allowed in the Yuv color space) is performed, the result is that luminance Y=48% is obtained.
However, when the color after the process in the Yuv color space is inversely mapped to the RGB color space, the result of B>100%, which cannot be displayed, is obtained, thus causing color clipping.